This invention relates to a process for the catalyzed oxidative alkoxycarbonylation of an amine or a substituted amine to form a carbamate and, more particularly, it relates to the elevated temperature and pressure oxidative alkoxycarbonylation of an amine or substituted amine in the presence of a palladium-based catalyst using carbon monoxide, an alkanol and an ioxygen-transfer agent to form an alkyl carbamate.
Alkyl carbamates, such as methyl N-phenylcarbamate, can be readily thermally decomposed to the corresponding isocyanate. If the structure of the material is such that two alkyl carbamate groups are present, thermal decomposition can yield a diisocyanate. A number of these diisocyanate compounds are useful monomers for making commercially useful polyurethanes. For example, toluene diisocyanate is an intermediate in the preparation of high performance polyurethane polymers.
The industrial preparation of isocyanates and polyisocyanates usually involves the use of phosgene, a chemical which has some undesirable features such as toxicity, cost and inclusion of undesirable halogen in the preparation process. The commercial process in use today involves treatment of the appropriate amine feedstock with phosgene. If alkyl carbonates could be made efficiently and cheaply, the use of phosgene in the manufacture of isocyanates could be avoided with consequent elimination of a substantial industrial hazard.
Recently, Asahi Chemical Industry has reported the conversion of amines to carbamates by oxidative carbonylation of an amine in the presence of catalytic amounts of palladium and iodine using an alcohol, oxygen and carbon monoxide. See S. Fukuoka et al., J. Chem. Soc. Chem. Commun. 399 (1984) and Chemtech 670-76 (1984). This method however has some limitations. The presence of elemental oxygen in contact with the organic mixture poses potential safety questions, and the amine starting materials can be hard to purify. So, a more easily purifiable starting material could be beneficial, as long as it forms the corresponding carbamate in high yield. In addition, other ways of supplying the necessary oxygen could answer the safety issue referred to above.
Now it has been found that several significantly broad classes of starting materials based upon amines can be effectively converted to carbamates by a process similar to that described by Fukuoka et al., and several oxygen transfer agents other than oxygen have been shown to convert these classes of materials as well as amines to the corresponding carbamates.